The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.

This provision was intended so that if the Soviet Union launched an attack against the European allies of the United States, it would be treated as if it was an attack on the United States itself. However the feared Soviet invasion of Europe never came. Instead, the provision was used for the first time in the treaty's history on September 12, 2001 in response to the September 11, 2001 Terrorist Attack.

Greece and Turkey joined the organization in February 1952. Germany joined as West Germany in 1955 and German unification in 1990 extended the membership to the areas of former East Germany. Spain was admitted on May 30, 1982 and the former Warsaw Pact Countries of Poland, Hungary and the Czech Republic made history by becoming members on March 12, 1999.

France is still a member of NATO but retired from the military command in 1966. Iceland, the sole member of NATO which does not have its own military force, joined on the condition that they would not be forced to participate in warfare.

The Soviet Union and its satellite states formed the Warsaw Pact in the 1950s in order to counterbalance NATO. Both organisations were opposing sides in the cold war. After the fall of the Iron Curtain in 1989, the Warsaw Pact disintegrated.

NATO saw its first military engagement in the Kosovo War, where it waged an 11-week bombing campaign against Serbian forces starting on March 24, 1999.

Charles de Gaulle's decision to remove France from NATO's military command in 1966 to pursue its own nuclear defence program precipitated the relocation of the NATO Headquarters from Paris, France to Brussels, Belgium by October 16, 1967. While the political headquarters is located in Brussels the military headquarters, the Supreme Headquarters Allied Powers Europe (SHAPE), are located just south of Brussles, in the town of Mons.

September 13, 2001, NATO invoked, for the first time in its history, an article in its charter that states that any attack on a member state is considered an attack against the entire alliance. This came in response to the September 11, 2001 Terrorist Attack/

On February 10, 2003 NATO faced a serious crisis because of France and Belgium breaking the procedure of silent approval concerning the timing of protective measures for Turkey in case of a possible war with Iraq. Germany did not use its right to break the procedure but said it supported the veto.

On April 16, 2003, NATO agreed to take command in August of the International Security Assistance Force (ISAF) in Afghanistan. The decision came at the request of Germany and the Netherlands, the two nations leading ISAF at the time of the agreement. It was approved unanimously by all 19 NATO ambassadors. This marked first time in NATO's history that it took charge of a mission outside the north Atlantic area. Canada had originally been slated to take over ISAF in August.